PROJECT SUMMARY Mitochondrial dysfunction plays a prominent role in the pathogenesis of Parkinson's disease (PD). Mitochondria are dynamics organelles that undergo continual fission and fusion events which serve crucial physiological function. Increasing evidence demonstrated abnormal mitochondrial dynamics in PD and PD models, suggesting that an altered balance in mitochondrial fission/fusion and impaired mitochondrial quality control was likely a common mechanism leading to mitochondrial and neuronal dysfunction/degeneration critical to the pathogenesis of PD. Mutations in VPS35 cause autosomal dominant PD. VPS35 is a key component of the retromer complex, which is be important for endosome-to-golgi and endosome-to-plasma membrane sorting and many signaling events. Recent studies found the localization of VPS35 on mitochondria and its involvement in inter-organelle communication between mitochondria and other organelles. In our preliminary studies, we confirmed the mitochondrial localization of VPS35 in both human neuroblastoma cells and human brain hippocampal neurons. We further found that overexpression of wild-type VPS35 in neurons caused significant changes of mitochondrial dynamics, which became more severe in neurons expressing PD- associated VPS35 mutant D620N. More importantly, we found that VPS35 physically interacted with DLP1, a key regulator of mitochondrial dynamics, which was enhanced by PD-associated mutation. All these exciting findings strongly suggest that VPS35 were involved in the regulation of mitochondrial dynamics which may be impaired by VPS35 PD associated mutations and detailed investigation into the potential role of PS1 in mitochondrial function and dynamics is warranted. Our proposed study will be the first mechanistic study investigating the effect of the pathogenic VPS35 PD mutations on mitochondrial dynamics/function and neuronal function and will likely reveal a novel role of VPS35 in the regulation of mitochondrial dynamics/function. In addition, our proposed studies will also provide novel insights into the contribution of retromer to various cellular processes and signaling pathways.